I'm doing a little literature study related to P2P networks, and found, to my surprise, that there has actually been some research on DC. Here's a list of publications that I've found on Google Scholar:

I. Bica and A. Furtunã, "DC++ and DDOS Attacks", 2007(?).PDF
This reads like it was written as an assignment for a bachelor course. Not of particularly high quality, but an interesting read nonetheless. (I chuckled at the "Nowadays it is being maintained and developed by various groups from the Internet.". Don't worry, I'm from the Internet!)

K. Molin, "Measurement and Analysis of the Direct Connect Peer-to-Peer File Sharing Network", 2009.PDF
Master Thesis, so somewhat more elaborate than a paper.

M.V. Bedekar and B. Deshpande, "Design and Implementation of a extensible Peer–to–Peer Content Recommendation System", 2010.PDF
Has actually been included in a journal (IJCSE), but isn't of particularly high quality.

And some that mention DC:

T. Karagiannis, A. Boido, et al. "File-sharing in the Internet: A characterization of P2P traffic in the backbone", 2003.PDF